Distributed antenna system for mimo signals

ABSTRACT

A distributed antenna system includes a multiple-input and multiple-output (MIMO) base station configured to output at least a first signal and a second signal. At least one master unit communicates with the MIMO base station. At least one remote unit communicates with the master unit. At least one antenna is coupled with the remote unit for receiving signals from the remote unit. A hybrid coupler is coupled between the remote unit and antenna and is configured to receive the first signal Ch 1  and the second signal Ch 2  from the remote unit on respective first and second ports and to provide an output signal on at least one output port. The output signal includes at least a portion of the first signal and at least a portion of the second signal. The antenna is coupled with the output port.

RELATED APPLICATIONS

This Application is a continuation-in-part of U.S. patent applicationSer. No. 12/634,212, filed Dec. 9, 2009, entitled “DISTRIBUTED ANTENNASYSTEM FOR MIMO SIGNALS”, the application and disclosure of which ishereby incorporated by reference in its entirety as though fullydisclosed herein.

This Application is also a continuation of International Application No.PCT/US2011/062640, filed Nov. 30, 2011, entitled “DISTRIBUTED ANTENNASYSTEM FOR MIMO SIGNALS”, which in turn, claims priority to ItalianPatent Application No. 2010A000714, filed Dec. 1, 2010, entitled“DISTRIBUTED ANTENNA SYSTEM FOR MIMO SIGNALS”, the applications anddisclosures of which are hereby incorporated by reference in theirentireties as though fully disclosed herein.

FIELD OF THE INVENTION

Embodiments of the invention are directed to wireless communicationsystems, and specifically directed to a distributed antenna system for awireless MIMO communications.

BACKGROUND OF THE INVENTION

A contemporary wireless communication system, such as distributedantenna system 10, is shown in FIG. 1, and includes a number of remoteunits 12 distributed to provide coverage within a service area of thesystem 10. In particular, each remote antenna unit 12 typically includesan antenna 14 and suitable electronics. Each remote unit is coupled to amaster unit 16. Each master unit 16 is, in turn, coupled to a RFcombination network 18 that combines the signals from at least onesingle-input-and single-output (“SISO”) base transceiver station (“BTS,”or more simply, “base station”) 20 (hereinafter, “SISO BTS” 20). Thesystem 10 may further include a system controller 22 to control theoperation of each master unit 16. As illustrated in FIG. 1, the system10 may include a plurality of master units 16 and a plurality of SISOBTSs 20, each master unit 16 configured to provide a combination of thesignals from at least two SISO BTSs 20 to its respective remote units12.

As illustrated in FIG. 1, each remote unit 12 may broadcast a wirelesssignal 24 that, in turn, may be received by a wireless device 26 thatmay be a mobile device, such as a telephone device or a computingdevice. In particular, and as discussed above, the wireless signal 24from each remote unit 12 may be a combination of signals from the atleast two SISO BTSs 20. Thus, the wireless device 26 may communicatewith the system 10 through any of the wireless signals 24 from theremote units 12.

To improve wireless communications, such as communications from a basestation to mobile devices, Multiple-Input/Multiple-Output (“MIMO”)technology might be utilized to provide advanced solutions forperformance enhancement and broadband wireless communication systems.Through various information series studies, it has been shown thatsubstantial improvements may be realized utilizing a MIMO technique withrespect to the traditional SISO systems. MIMO systems have capabilitiesthat allow them to fully exploit the multi-path richness of a wirelesschannel. This is in contrast with traditional techniques that try tocounteract multi-path effects rather than embrace them. MIMO systemsgenerally rely upon multi-element antennas at both of the ends of thecommunication links, such as in the base station and also in the mobiledevice. In addition to desirable beam-forming and diversitycharacteristics, MIMO systems also may provide multiplexing gain, whichallows multi data streams to be transmitted over spatially-independentparallel sub-channels. This may lead to a significant increase in thesystem capacity. Generally, the systems illustrated in FIG. 1 cannottake advantage of MIMO technology.

For example, the wireless device 26 of FIG. 1 communicates with only oneof the remote units 12, though it may be in the range of a plurality ofremote units 12. The wireless signals 24 from each remote unit aretypically at the same frequency and carry the same data, andcommunication between a plurality of remote units 12 and the wirelessdevice 26 simultaneously may result in signal degradation andcollisions. Moreover, data bandwidth from the wireless device 26 isconstricted to the speed of reception and processing of data from oneremote unit 12.

It is therefore, desirable to take advantage of MIMO signals within awireless system, such as distributed antenna system, without requiringan entirely new system to be installed for handling MIMO signals.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a distributed antenna system(“DAS”) and methods of use that can be used to provide a multiple-inputand multiple-output (“MIMO”) mode of operation. In particular, someembodiments include a MIMO base station configured to output at least afirst signal and a second signal and a hybrid coupler coupled to reviewthe first and second signals. The coupler is configured to receive thefirst signal and the second signals on respective first and second portsand provide an output signal on output ports. The output signals includeat least a portion of the first signal and at least a portion of thesecond signal. The system further includes at least one master unitcommunicating, and at least one remote unit communicating with themaster unit and configured to communicate at least the output signals toa device, such as a customer's wireless device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a contemporary distributed antenna system.

FIG. 2A is a block diagram of a distributed antenna system consistentwith embodiments of the invention.

FIG. 2B is a block diagram of a distributed antenna system consistentwith embodiments of the invention.

FIG. 3 is a block diagram of a distributed antenna system consistentwith embodiments of the invention used with an indoor environment.

FIG. 4 is a detailed block diagram of a master unit utilized inembodiments of the invention.

FIGS. 5A and 5B are a detailed block diagram of a portion of a remoteunit utilized in embodiments of the invention.

FIG. 6 is a detailed block diagram of an alternate portion of a remoteunit utilized in embodiments of the invention.

FIG. 7A is a block diagram of MIMO BTS in an outdoor scenario.

FIG. 7B is a block diagram of an alternative distributed antenna systemconsistent with embodiments of the invention.

FIG. 8 is a block diagram of a 90° 3 dB hybrid coupler and transferfunction representation.

FIG. 9 is a block diagram of an overview of LTE physical channelprocessing.

FIG. 10 is a distributed antenna system for implementing the invention.

FIG. 11 is a block diagram of an alternative distributed antenna systemconsistent with embodiments of the invention.

FIGS. 12 is a block diagram of an alternative distributed antenna systemconsistent with embodiments of the invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of embodimentsof the invention. The specific design features of the system and/orsequence of operations as disclosed herein, including, for example,specific dimensions, orientations, locations, and shapes of variousillustrated components, will be determined in part by the particularintended application and use environment. Certain features of theillustrated embodiments may have been enlarged, distorted or otherwiserendered differently relative to others to facilitate visualization andclear understanding.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2A illustrates a schematic view of one possible implementation of aMIMO system, wherein a MIMO base station is incorporated with adistributed antenna system, such as that shown in FIG. 1. With respectto FIG. 1, like reference numerals are utilized in FIG. 2A whereapplicable. As illustrated, two SISO base stations 20 (each SISO basestation 20 a “SISO BTS” 20) are coupled with each of the remote units.Additionally, a MIMO base station 30, including antennas 31 and 32, arecoupled with the remote units 12. Antenna 1 of the MIMO BTS 30 iscoupled with remote units 12 a and 12 d through a first master unit 16(MASTER UNIT 1). Antenna 2 of the MIMO BTS 30 is coupled with remoteunits 12 b and 12 c through as second master unit 16 (MASTER UNIT 2). Assuch, as illustrated by the wireless signals 24 produced at each remoteunit, each master unit will transmit signals from the MIMO BTS 30, inaddition to a combined signal from the combination of signals output bythe SISO BTSs 20. However, because each antenna is not coupled to allthe remote units, each remote unit will only transmit one of the twoavailable MIMO signals as shown. The respective wave fronts areillustrated corresponding to the feed or connection lines from each ofthe appropriate antennas 31, 32 in FIG. 2A.

While the embodiment illustrated in FIG. 2A may be utilized to providethe availability of MIMO signals within a distributed antenna system,such a system may not realize all of the desired performanceimprovements associated with a MIMO system. For example, even ifwireless device 26 receives all the MIMO signals from a combination ofat least two of the remote units, there may be a received RF powerimbalance because the wireless device 26 might be located much closer toone remote unit 12 than to another. Furthermore, accordingly to wirelessstandards that support MIMO features, there are some signalingparameters, such as the WiMAX Frame Preamble or the LTE PrimarySynchronization Signal (“P-SS”), which are, or actually can be,transmitted by only one of the MIMO BTS antennas 31, 32. Therefore, in aMIMO system, as in FIG. 2A, wherein these signals are not transmitted byall of the remote units, the system may not be sufficiently reliableunless there is a very high level of coverage redundancy/overlap betweenthe remote units. In operation, dynamic switching between SISO and MIMOoperating modes may present performance problems.

FIG. 2B illustrates a schematic view of another possible implementationof a distributed antenna system 40 that incorporates MIMO featuresconsistent with embodiments of the invention. System 40 includes atleast one remote unit 42 to provide coverage in a service area. Inparticular, system 40 includes a plurality of remote units 42 a-h. Eachremote unit 42 a-h includes at least one coverage antenna 44 and iscoupled to a master unit 46 a-b. Each master unit 46 a-b, in turn, iscoupled to a respective summing circuit 48 a-b that may be configured tocombine at least two inputs. In particular, each summing circuit 48 a-bcombines a signal from an RF combination network 50 coupled to SISO BTSs54 a and 54 b with a signal from the MIMO BTS 58. The signals 68, 70from the MIMO base station are presented through a hybrid coupler 52coupled with the signals from the antennas 31 and 32 of the MIMO BTS 58.The RF combination network 50 is coupled to a plurality of SISO BTSs 54a-b and outputs at least one combined SISO BTS signal as at 56 a-b.

In one aspect of the present invention, a hybrid coupler 52 is coupledto the MIMO BTS 58 to cross-couple all MIMO signals (in the exampleillustrated that is two MIMO signals) to each of the remote units 42.Therefore, each of the remote units 42 transmits all of the MIMO BTS 58data streams, as well as the combined data streams from the SISO BTSs54. The hybrid coupler 52 is configured to receive at least two MIMOsignals from the respective antennas 31 and 32 on respective first andsecond ports (Ports 1 and 2 as illustrated in FIGS. 2A-B), and providean output signal on at least one output port (Ports 3 and 4 asillustrated in FIGS. 2A-B). In the illustrated embodiment, combinedsignals from the two MIMO BTS antennas 31 and 32 are provided at theoutput ports 3 and 4. Each output signal includes at least a portion ofthe first signal from antenna 31, and at least a portion of the secondsignal from antenna 32. In the hybrid coupler circuit, the portion ofthe first signal (e.g., Antenna 1) or the portion of the second signal(e.g., Antenna 2) presented at one of the input ports 1, 2 is phaseshifted with respect to the first signal and/or second signal receivedat the respective other of first and second coupler ports 1, 2. Inparticular, the hybrid coupler 52 is disposed between a MIMO BTS 58 andmaster stations 46 a-b such that the hybrid coupler 52 is configured toreceive the first and second signals from the MIMO BTS 58, including afirst signal at 62 from a first MIMO antenna 31, and a second signal at64 from a second MIMO antenna. In turn, the hybrid coupler 52 combines aportion of the first signal 62 with a phase shifted portion of thesecond signal 64 and outputs that first output signal at 68 on a firstoutput port (e.g., output port 3). Coupler 52 also combines a portion ofthe second signal 64 with a phase shifted portion of the first signal 62and outputs that second output signal at 70 on a second output port(e.g., output port 4). In one exemplary embodiment, the hybrid coupler52 is a 90° 3 dB coupler (also referred to as a “quadrature” coupler).

Furthermore, it will be appreciated that, in some embodiments, the firstand second signals from the MIMO BTS 58 may be separately provided torespective summing circuits 48 a-b and/or master units 46 a-b ratherthan passing through hybrid coupler 58, such as the embodiment shown inFIG. 2A.

In some embodiments, the summing circuit 48 a is configured to provide afirst master unit signal 72 that is a combination of the combined SISOBTS signal 56 a and the first combined MIMO signal 68. Summing circuit48 b is configured to provide a second master unit signal 74 that is acombination of the combined SISO BTS signal 56 b and the second combinedMIMO signal 70. The master units 46 a-b and remote units 42 a-h, inturn, may be controlled by a system controller 76, which may provideoverall supervision and control of the master units 46 a-b and remoteunits 42 a-h, as well as alarm forwarding.

In some embodiments, each remote unit 42 a-h may be connected to theirrespective master units 46 a-b via high speed digital transport mediumsor links 80 a-b, 82, 84 a-b and/or 86 a-b. Alternatively, an analogtransport medium/link might be used for connecting the remote units withrespective master units. Also, the transport links might be implementedas optical links using optical fiber as discussed below. With suchfiber, the traffic between the remote units and master units might beimplemented using a radio-over-fiber (RoF) format. In this manner, thefirst master unit signal 72 and/or the second master unit signal 74 areprovided to at least a portion of the remote units 42 a-h in a digitalformat, which may assist in preventing at least some degradation due totransmission line effects. It will be appreciated by one having ordinaryskill in the art that filtering may also be used to allow and/or preventthe distribution of specific signals. As such, and in some embodiments,each of the links 80 a-b, 82, 84 a-b and/or 86 a-b may be a widebanddigitally modulated optical interface, such as fiber optic cable. Thus,each master unit 46 a and/or 46 b may be configured to digitize theirrespective master unit signals 72 and/or 74 and output those digitalsignals for their respective remote units 42 a-42 d and/or 42 e-h. Thesedigital output signals may, in some embodiments, be time divisionmultiplexed into frames and converted into a serial stream. The remoteunits 42 a-42 d and/or 42 e-h, in turn, may be configured to receive thedigital output signals from their respective master units 46 a and/or 46b, convert the digital output signals into electrical signals, ifnecessary de-frame various time slots and/or de-serialize the electricalsignals, and transmit the electrical signals via their local antenna 44to at least one wireless unit 90.

The remote units 42 a-h are configured to send and/or receive digital RFvoice and/or data signals to and/or from a wireless unit 90 via theirlocal antennas 44. As discussed below, depending on how the remote unitsare coupled to the master units, the remote units 42 b, 42 d and/or 42 fmay also be configured to receive a digital signal from remote units 42a, 42 c and/or 42 e, respectively, which precede it in a chain. Thisdigital signal between remote units may contain signals from thewireless unit 90 received by the preceding remote units 42 a, 42 cand/or 42 e. The digital signal may then be combined with another signalreceived by the remote units 42 a, 42 c and/or 42 e. As such, digitalwireless signals from wireless units 90 may be combined and/ortransmitted back to a respective master unit 46 a and/or 46 b. Themaster units 46 a and/or 46 b may then convert a signal from itsrespective remote units 42 a-d and/or 42 e-h from an optical signal toan electrical signal and send the electrical signal to the SISO BTSs 54a-b and MIMO BTS 58, which may be configured to detect and receive theirrespective portions thereof. Alternatively, the master units 46 a and/or46 b may then convert a signal from its respective remote units 42 a-dand/or 42 e-h from an optical signal to an electrical signal, separatethe electrical signal into a plurality of electrical signals in aplurality of bands corresponding to those utilized by the SISO BTSs 54a-b and MIMO BTS 58, convert the plurality of electrical signals into aplurality of analog signals, and send the plurality of analog signals tothe SISO BTSs 54 a-b and/or MIMO BTS 58.

As illustrated in FIG. 2B, by way of example, a master unit 46 a-b maybe selectively connected to respective remote units 42 a-h in a numberof ways. For example, master unit 46 a is illustrated as connected toremote units 42 a-b through half-duplex link 80 a for uplink to theremote units 42 a-b and half-duplex link 80 b for downlink. However,master unit 46 a is illustrated as connected to remote units 42 c-dthrough full-duplex link 82. Similarly, master unit 46 b is illustratedas connected to remote units 42 e-f through half-duplex link 84 a foruplink to the remote units 42 e-f and half-duplex link 84 b fordownlink. However, master unit 46 b is illustrated as connected toremote unit 42 g through full-duplex link 86 a and connected to remoteunit 42 h through full-duplex link 86 b. As such, in a full-duplex link,the uplink signals and downlink signals are carried on differentwavelengths and a wavelength division multiplexer (“WDM”) is employed tocombine and/or split the two optical signals at the master units 46 a-band remote units 42 a-h. Alternatively, the master units 46 a-b andremote units 42 a-h may communicate through a different transceiver forhigh data rate media such as coax cable, twisted pair copper wires, freespace RF or optics, or shared networks such as Ethernet, SONET, SDH, ATMand/or PDH, among others. As will be appreciated, one or more of theexemplary links, as illustrated in FIG. 2B, might be selected forcoupling all of the remote units to the master units.

In some embodiments, the system 40 illustrated in FIG. 2B may beselectively and dynamically utilized as a SISO system and/or a MIMOsystem. For example, if the hybrid coupler 52 is not activated, thesignals from the SISO BTSs 54 a-b may be transmitted to at least aportion of the remote units 42 a-h and the system may be utilizedsimilarly to a SISO system. In this manner, each of the remote units 42a-h communicates through at least two wireless frequencies thatcorrespond to those used by the SISO BTSs 54 a-b. However, when thehybrid coupler 52 is selectively activated, the signals from the SISOBTSs 54 a-b may be combined with the combined MIMO output signals 68, 70such that each remote unit 42 a-h communicates the signals from the SISOBTSs 54 a-b through at least two wireless frequencies that correspond tothose used by the SISO BTSs 54 a-b and communicates both or all of theMIMO signals. Thus, selective activation of the hybrid coupler 52results in dynamically reconfiguring the system 40 from a SISO mode ofoperation to a MIMO mode of operation. As such, the system 40 may beused as an indoor MIMO system that is configured to handle a WiMAX FramePreamble and/or LTE P-SS (Primary Synchronization Signal) that eitherare, or optionally can be, transmitted by only one of the MIMO BTSantennas.

Thus, portions of the first and second signals 62 and 64 from the MIMOBTS 58 may be cross-coupled and combined and sent to all the remoteunits 42 a-h without affecting the MIMO operation thereof. For example,each remote unit 42 a-h of the system 40 may be configured to transmitboth (or all) data streams from the MIMO BTS 58 and its antennas 31, 32(e.g., the output signal 68 or the output signal 70) along with thecombined SISO BTS signals 56 a-b.

FIG. 3 illustrates a schematic view of a wireless communication system100 similar to the system 40 of FIG. 2B, but that includes a pluralityof remote units 42 a-d used in an indoor environment 104. In particular,FIG. 3 illustrates that remote units 42 a-b coupled to the master unit46 a may be positioned about the indoor environment 104 such that theirsignals are not substantially overlapping. Remote units 42 c-d may besimilarly positioned. As such, a wireless device 90 in a portion of theenvironment 104 may be able to receive signals from two remote units (asillustrated in FIG. 3, wireless device 90 receives signals from remoteunits 42 a and 42 d). As such, MIMO spatial multiplexing may beexploited, as the wireless device 90 is capable of receiving twonon-identical signals from two remote units 42 a and 42 d fed by twodifferent master units 46 a and 46 b, respectively.

Referring to FIG. 3, by incorporating a hybrid coupler in thedistributed antenna system in accordance with the invention, all of theMIMO signals (in this case, both MIMO signals) can be cross-coupled andsent to all of the remote units without affecting the MIMO operation.Each remote unit can transmit both the MIMO parallel data streamswithout producing inter-stream interference because there are 90° ofphase shifting between them. That is, a distributed MIMO concept issplit in two parallel distributed MIMO systems. The first one is “inphase”, while the second one is “90° phase shifted”. Of course, in orderto exploit the MIMO spatial multiplexing, it is necessary that thewireless device 90 receive substantial power contributions from at leasttwo of the remote units that are fed by different master units.Therefore, as such, it is desirable that the wireless device 90 receivespower from more than one of the two remote units, for example 42 a, 42d, that are coupled with different master units, in order to maintainMIMO spatial multiplexing.

One benefit of the invention is that it solves problems noted abovewherein the remote units transmit signals associated with only one ofthe MIMO base station antennas. Similarly, performance impairmentsbetween transmitted parallel data streams that may affect the wirelessunit 90 when located closer to a specific remote unit 42 a-d may beaddressed, as received power levels for two signals from two remoteunits 42 a-d are typically similar for most locations in the indoorenvironment 104, thus increasing data throughput. This issue is oftenreferred to as the “near-far problem” affecting a distributed MIMOsystem with remote units transmitting only a single data stream. Thisissue is addressed as discussed herein below using a 3 dB 90° Hybridcoupler.

Another particular benefit of the present invention is the ability toprovide deployment of a MIMO system within an existing distributedantenna infrastructure that is originally implemented for a SISO system.The present invention may also operate with a selective coupling ordynamic switching between a SISO and a MIMO operation mode that isperformed by a MIMO base station. Furthermore, when the MIMO basestation operates in downlink spatial multiplexing mode, the inventionprovides the performance equalization related to the transmittedparallel data streams. That is, as noted, the 90° 3 dB hybrid coupler isused in order to solve the “near-far problem”. The inter-streamcross-coupling performed through the Hybrid Coupler acts similarly to oras a substitute for the MIMO pre-coding as specified by the 3GPP LTEstandard in order to address the potential mismatch in performancebetween the two data streams. That is, the pre-coding provided by theinvention is intended to equalize the performance (like bit error rate,error vector magnitude, etc.) of two data streams experiencing differentchannel conditions. In case of the “near-far problem” the two streamsexperience different channel path-losses. Furthermore for a properoperation of the LTE standard, it is mandatory that the pre-codingcoding scheme is orthogonal so that the original symbols can berecovered at the receiver avoiding inter-stream interference. Thiscondition is met by the 90° Hybrid Coupler input-output transferfunction as discussed below in accordance with one aspect of theinvention.

With reference to FIG. 8, a 90° 3 dB hybrid coupler is shown and acts asa “hardware” MIMO pre-coding circuit to compensate for possibleperformance impairment between the data streams (code-words) due to the“near-far problem”. The equation shown in FIG. 8 illustrates the inputand output port relationship and the transfer function matrix of a 90° 3dB hybrid coupler, respectively. As such, in accordance with an aspectof the invention, the transfer function matrix reflected in FIG. 8 canalso be regarded as the MIMO pre-coding matrix of the 90° 3 dB hybridcoupler. FIG. 9 illustrates the location of the MIMO pre-coding block ina typical LTE physical channel processing stream. In accordance with anaspect of the present invention, the exploitation of a 90° 3 dB hybridcoupler into the LTE MIMO distributed antenna system as disclosed hereinlocates the performance of the pre-coding at the BTS antenna portsrather than in the BTS physical channel processing. Therefore theinvention also represents a hardware improvement to the MIMO BTSscheduler circuitry which is responsible for pre-coding selection on theUser Equipment's feedback basis.

In accordance with another aspect of the present invention, the hybridcoupler that is utilized in embodiments of the invention makes inputsignals orthogonal to each other. The device's reciprocity between theinput ports 1, 2 and the output ports 3, 4 provides that the resultingtransfer function matrix remains the same, even exchanging the input andoutput ports. This provides the invention with the ability to combineMIMO signals without affecting their capability to support spatialmultiplexing.

FIGS. 4-6 illustrates an exemplary distributed antenna system forimplementing embodiments of the invention. Focusing now on a master unit46, FIG. 4 contains a detailed block diagram of the master unit 46. Eachmaster unit 46 may contain from one to six radio channels (hereinafterreferred to as a “path”) 110, from one to four digitally modulatedoptical channels 112, a controller 114, a clock generator 116, and anEthernet switch 118.

In one embodiment, each path, such as 110 a, may be configured to handlea signal to and from SISO BTSs 54 a-b and/or MIMO BTS 58, for example.For a FDD air interface, the paths 110 a employ a combiner and aduplexer 120 to handle the uplink signal and the downlink signal. An RFdownconverter 122 may amplify the received signal from thecombiner/duplexer 120 to ensure that an A/D converter 124 is fullyloaded. The RF downconverter 122 sets a center frequency of a bandwithin the A/D converter pass band. The wideband A/D 124 digitizes theentire downlink band of the air interface to ensure all downlinkchannels are digitized. A resampler 126 converts the signal to a complexformat, digitally downconverts the frequency band in some cases,decimates and filters the signal, and resamples it. This reduces theamount of data associated with a downlink signal, such as 128 a, thathas to be transferred over the optical lines and synchronizes the rateof the digitized data to the optical network bit rate.

The uplink section of the radio channel 110 a sums 120 the uplinksignals, such as signals 129 a-d, for its assigned band from remoteunits 42 coupled to the master unit 46 after they are converted to anelectrical signal. The summation 130 is resampled, interpolated tochange to a different data rate in some cases, and upconverted by theresampler 132 and then converted to an analog form by the D/A converter134. The RF upconverter 136 translates the center frequency of theanalog signal to the appropriate frequency for the air interface andamplifies it. The amplified signal is applied to the combiner/duplexer120 and is routed back to the SISO BTSs 54 a-b and/or MIMO BTS 58.

In embodiments utilizing TDD air interfaces, the combiner and duplexerare replaced by a switching function 138 shown in FIG. 4 for example inradio channel 110 b and detailed in FIG. 5. While the master unit 46 isreceiving the downlink signal, a RF amplifier in the RF upconverter isdisabled and a shunt switch in the switching function 138 may shunt theRF amplifier to ground to further reduce leakage. During intervals whenthe master unit 46 is sending the uplink signal to the base station 42,the RF amplifier is enabled, the shunt switch is opened and a seriesswitch in the switching function 138 may be opened to protect the RFdownconverter from damage due to high power levels. The switch controltiming 144 is determined by a master unit controller 114 from thedownlink signal 128 b. Additionally, a formatter 146 may apply a datacompression to reduce the redundant digital information included in theserial data stream before it is sent to the transmitter in anelectro-optical transceiver 148. The compression may allow for savingbandwidth or for using a less costly transceiver with lower bit rate.The compressed serial data may be converted into an uncompressed datastream after being received on the opposite ends in the optical receivedof 148 by the receiver side formatter 146.

Each digitally modulated optical channel 112 a-b is composed of aformatter 146 and an electro-optical transceiver 148. On the outgoingside, the formatter 146 blocks, into time division multiplexed frames,the digitized downlink signal 128 a-b along with a customer Ethernet inReduced Media Independent Interface (“RMII”) format 150 a-b, operationand maintenance (“O&M”) data 152 a-c and synchronization information. Inother embodiments, other interfaces such as MII, RMII, GMII, SGMII,XGMII, among others may be used in place of the RMII interface. Theframed data may be randomized by exclusive or'ing (XOR) it with theoutput of a linear feedback shift register to remove long strings oflogic ones or zeros. Other known coding formats such as 8 bit/10 bit or64 bit/66 bit coding may also be used, but may result in a decrease inefficiency in the use of the digital serial link. This digital data isthen converted to a serial stream which is used to modulate an opticaltransmitter within the electro-optical transceiver 148. In a singlefiber implementation, a wavelength division multiplexer (“WDM”) 149 maybe employed to combine or split the two optical signals.

For incoming signals from the remote units 44, the electro-opticaltransceiver 148 converts the optical signal to an electrical signal. Theformatter 146 phaselocks to the incoming bit stream and generates a bitclock that is phaselocked to the data rate and aligned with the serialdata stream. The formatter 146 then converts the serial stream to aparallel digital data stream, de-randomizes it and performs framesynchronization. It then breaks out the digitized uplink signal for eachband, buffers each band and routes the bands to the appropriate radiochannel 110 a, 110 b, if necessary. Finally, the formatter 146 breaksout the buffers and O&M Ethernet data 152 a-c and the user Ethernet data150 a-b and routes them to the controller 114 and the Ethernet switch118, respectively.

The master unit controller 114 uses locally stored information andinformation from the O&M Ethernet data to configure and control theother blocks in the master unit 46. It also passes this information tothe remote units 42 and reports status of the remote units 42 and themaster unit 46 to the system controller 76. When a radio channel, suchas 110 b, is assigned to a TDD air interface, the master unit controller114 also uses the corresponding downlink signal 128 b to derive TDDswitch control timing 144.

The system controller 76 generally has overall system control. Themaster unit controller 114 functions to configure individual modules aswell as supervise individual modules. As part of the configuration andsupervision functions, the master unit controller 114 is operable todetermine the uplink/downlink switch timing in TDD systems by decodingthe downlink signaling or acquiring it from a different source such asthe time variant UL RSSI, or some base station clock signal providedfrom an external source. The downlink frame clock in TDMA systems may bedetermined and distributed by decoding the downlink signaling to allowtime slot based functions such as uplink or downlink muting, uplink ordownlink Received Signal Strength Indication (“RSSI”) measurementswithin time slots, uplink and downlink traffic analysis, etc. The masterunit controller 114 may detect active channels in the RF spectrum toassist in or automatically configure the filter configuration in theresampler 126, 132. Optimal leveling of the individual signals in theresampler may also be determined by measurement of the RSSI of thevarious signals in the downlink RF band. A remote unit controller mayperform similar tasks in the uplink of the remote unit 42.

The clock generator 116 may use a stable temperature compensated voltagecontrolled crystal (“TCVXO”) to generate stable clocks and referencesignals 154 for master unit 46 functional blocks. Although, one ofordinary skill in the art will appreciate that other devices or crystalsmay also be used to generate clocking signals as long as they arecapable of producing the stable clocks required by the system.

Focusing now on a remote unit 42, FIG. 5A and FIG. 5B contain a detailedblock diagram of a remote unit 42 consistent with embodiments of theinvention. Each unit 44 may contain from one to six radio channels 160,one or two DMOCs 162, a remote unit controller 164 and an Ethernetswitch 166.

The DMOCs 162 may be designated as the downstream 168 and upstreamchannels 170. The downstream channel 168 is connected to a remote unit42 that precedes this remote unit 42 in a daisy chain, if so configured.The upstream channel 170 is connected to a master unit 46 or anotherremote unit 42. The DMOC 162 functional blocks are similar to those inthe master unit 46. Both consist of a formatter 172 and electro-opticaltransceiver 174. Outgoing data is buffered, formatted into frames,randomized, parallel to serial converted and used to modulate an opticaltransmitter in the electro-optical transceiver 174. Incoming data isconverted from an optical to electrical format, bit synchronized,de-randomized, frame synchronized and converted to a parallel format.The various data types are then broken out buffered and distributed toother function blocks within the remote unit 42. In some embodiments,formatter 172 may implement compression and decompression schemes toreduce bandwidth over the digital optical link.

Radio channels in the remote unit 42 are functionally similar to thosein the master unit 46. Each radio channel is configured to handle asingle RF band. Unlike the master unit 46 radio channels 110, the remoteunit 42 radio channels 160 are connected via a cross band coupler 176 toits antenna 44. For FDD air interfaces, the radio channels, such asradio channel 160 a, employ a duplexer 178 to split the uplink and thedownlink signal. Duplexers, cross-band combiners and couplers may beoptional for some embodiments of either the master unit 46 or remoteunits 42. In these embodiments, additional antennas may replace theduplexer 178 and cross-coupler 176 in the remote units 42. Extra cableswould be required in the master unit 46. A RF downconverter 180amplifies the received uplink signal from the antenna 44 to ensure an NDconverter 182 is fully loaded and sets the center frequency of the bandwithin the A/D converter pass band. The wideband ND 182 digitizes theentire uplink band of the air interface to ensure all uplink channelsare digitized. A resampler 184 converts the uplink signal to a complexformat, digitally downconverts the signal in some cases, decimates andfilters the signal, and resamples it with a multi-rate filter bank. Thisreduces the amount of data that has to be transferred over the opticallinks and synchronizes the rate of the digitized data to the opticalnetwork bit rate. The output of the resampler 184 is added to the uplinksignals 186 a from the downstream remote units 42 in summer 187. Thesummed uplink signal 188 a for each band is then sent to a formatter 172in the upstream channel 170 in the DMOC 162.

The downlink signal 190 for each band (190 a, 190 b) is interpolated andfrequency shifted in the resampler 192. The group delay of individualspectral components can be adjusted via filters or delay elements in theresampler 192. The signal is then converted to an analog form by the D/Aconverter 194. The RF upconverter 196 translates the center frequency ofthe analog downlink band to the appropriate frequency for the airinterface and amplifies it. The amplified signal is then applied to theantenna 44 and transmitted to a wireless unit 90.

For TDD air interfaces, the duplexer 178 is replaced by the switchingfunction 138 shown in radio channel 160 b and FIG. 5A. While the remoteunit 42 is receiving the uplink, the RF power amplifier in the RFupconverter 196 is disabled and a shunt switch in the switching function138 shunts the RF power amplifier to ground to further reduce leakage.When the remote unit 42 is transmitting the downlink signal, the RFpower amplifier is enabled, the shunt switch is opened to permit thedownlink signal to reach the antenna 44 and a series switch in theswitching function 138 is opened to protect the RF downconverter 180from damage due to high power levels. As with the master unit 46, theswitch control timing 144 is determined by the controller 164 from thedownlink signal 190 a, 190 b.

The clock generator 198 includes a voltage-controlled crystal oscillator(“VCXO”) that is phaselocked to the incoming serial data stream bit ratevia a narrowband phaselocked loop (“PLL”). The VCXO output is split andis used as the frequency reference 200 for the local oscillators in eachradio channel 160 a-b, the sampling clocks for the ND 182 and D/A 194converters, and a clock for the other blocks in the remote unit 42. Oneof ordinary skill in the art will realize that the long term frequencyaccuracy should be good to ensure the local oscillators are on frequencyand that the short term jitter levels should also be low to ensure thatthe jitter does not corrupt the A/D and D/A conversion processes. Byphaselocking to the data rate of the optical link, which is derived fromthe stable TCVCXO in the master unit 46, the remote unit 42 does notrequire an expensive oven compensated oscillator or a GPS discipliningscheme to maintain long term frequency accuracy, thereby, making themore numerous remote units 42 less expensive. The use of a narrow bandPLL and a crystal controlled oscillator may assist in reducing shortterm jitter for the ND and D/A converter clocks. Using the recovered,jitter reduced clocks 202 to re-clock the transmit data in the opticallinks at each remote unit 42 reduces jitter accumulation which mayassist in improving A/D and D/A converter clocks in the downstreamremote units 42 and may assist in reducing the bit error rate (“BER”) ofthe optical communication channels 162.

The remote unit controller (“RUC”) 164 uses locally stored informationand information from the O&M Ethernet to configure and control the otherblocks in the remote unit 42. Downstream RMII 152 d and upstream RMII152 e may also be supplied to the formatter 172. In addition, local O&Mdata 206 may be configured at a local O&M terminal 204. Remote unit 42also passes this information to the up and downstream remote units 42and/or master unit 46. The RUC 164 additionally uses the appropriatedownlink signal to derive TDD switch control timing 144 when required.

In an alternate embodiment of the radio channel 160 c utilized in aremote unit 42, the radio channel 160 c may also employ digitalpre-distortion to linearize the power amplifier. This embodiment of theradio channel 160 c in a remote unit 42 is shown in the block diagramsof FIG. 6. In this embodiment, a third signal path may be added to oneor more radio channels 160c. The third path couples off the downlinksignal after power amplification and digitizes it. The signal from theantenna 44 is received in an RF downconverter 208, which amplifies thereceived signal to ensure an A/D converter 210 is fully loaded and setsthe center frequency of the band within the A/D converter pass band. Thewideband A/D 210 digitizes the entire uplink band of the air interfaceto ensure all uplink channels are digitized. The digitized signal iscompared to a delayed version of the downlink signal in the digitalpre-distortion unit 212 and the difference is used to adaptively adjustthe gain and the phase of the signal prior to D/A conversion to correctfor non-linearity in the power amplifier.

While the present invention has been illustrated by the description ofthe embodiments thereof, and while the embodiments have been describedin considerable detail, it is not the intention of the applicant torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. For example, a distributed antenna systemconsistent with embodiments of the invention may have more or fewerremote units 42, master units 46, summing circuits 48, RF combinationnetworks 50, hybrid couplers 52, SISO BTSs 54, MIMO BTSs 58 and/orsystem controllers 76 than those illustrated. In particular, each MIMOBTS 58 may include more or fewer output ports 62 and/or 64.

Additionally, each master unit 46 may be connected to more or fewerremote units 42 than those illustrated. As such, a plurality of remoteunits 42 may be connected to each master unit 46 through two linksand/or along a single link. Alternatively, each remote unit 42 may beconnected to a master unit 46 through a dedicated link. In someembodiments, up to six remote units 42 may be connected in series from amaster unit 46. As such, remote units 42 may be positioned to optimizecoverage within a coverage area.

Furthermore, system 40 and/or 100 may not include summing circuits 48a-48 b. As such, the master unit 46 a may combine the combined SISO BTSsignal 56 a and first output signal 68, while the master unit 46 b maycombine the combined SISO BTS signal 56 b and second output signal 70.Additionally, the system 40 may also not include RF combination network50. As such, the master unit 46 a may combine one or more signals fromthe SISO BTSs 54 and the first output signal 68, while the master unit46 b may combine one or more signals from the SISO BTSs 54 and thesecond output signal 70.

Moreover, and in some embodiments, the master unit controller 114 maymeasure a pilot signal strength of CDMA or Orthogonal Frequency-DivisionMultiplexing (“OFDM”) signals to properly set the level of the downlinksignals, as the RSSI can vary at different capacity loading. The pilotsignals generally remain constant with a configured ratio between pilotlevel and a maximum composite for full loading, the required headroomfor the signals may be maintained. The master unit controller 114 mayalso measure and supervise the signal quality of the provided downlinkchannels. In case of signal degradation, an alarm may be set and theoperator can focus on a base station (e.g., SISO or MIMO BTS) withouthaving to troubleshoot the entire system 40 and/or 100.

In some embodiments, the master unit controller 114 determines theamount of channels for a narrowband base station standard such as GlobalSystem for Mobile communications (“GSM”). Together with the measurementof the Broadcast Control Channel (“BCCH”), which is constant in power,the proper headroom that is required for a multichannel subband may bedetermined and overdrive or underdrive conditions may be avoided. Inother embodiments, the master unit controller 114 monitors the crestfactor of a transmitted spectrum in the presence of multiple channels.The crest factor may provide input to the leveling of the transmit poweror the power back-off of particular gain stages of the system. Theconfigured headroom is generally higher than the measured crest factorto avoid signal degradation due to clipping or distortion. In addition,a crest factor reduction mechanism may be employed in the resampler insome of the embodiments to reduce the crest factor and make moreefficient use of the RF power amplifier in the remote unit 42 or assistin reducing the number of required bits per sample that need to betransmitted over the link.

As illustrated in FIGS. 7A and 7B, the invention provides benefits inregard to the Uplink path of a MIMO communication system. Both WiMAX andLTE wireless standards encompass Uplink MIMO features. In particular the“Uplink Collaborative MIMO” is implemented in Mobile WiMAX, while“Uplink Multi-User MIMO” is the term adopted in LTE for indicating thesame technique. The peculiarity of this MIMO scheme is to increase thetotal Uplink sector capacity by reusing time/frequency resourcesallocated to the different UEs (User Equipments) or mobile devices,rather than to boost the data rate per single user as for DownlinkSingle-User MIMO (Spatial Multiplexing).

FIG. 7A shows a MIMO BTS 300 in an outdoor scenario, which cancoordinate the data reception from two different mobile devices A and B,equipped with a single transmitter (Tx) antenna each, and then allocatethe same time/frequency resources to them. The decoding of theirrespective data streams is performed by the BTS through the same signalprocessing as for a Single-User MIMO situation. That is, the two datastreams belonging to spatially separated users, rather than to a singleuser with two co-located Tx antennas, are spatially multiplexed. As aconsequence the saved time/frequency resources can be allocated to moreusers in order to increase the total Uplink sector capacity. Finally theMIMO transmission might benefit from the fact that the two transmittersare largely separated leading to a consequent increase of theprobability to have uncorrelated radio channels which is one importantrequirement for successful MIMO operation.

FIG. 7B highlights such a potential benefit in an indoor system. FIG. 7Billustrates a schematic view of at least a portion of a wirelesscommunication system 220 somewhat similar to the system 100 of FIG. 3,but that does not show the plurality of SISO BTSs 54 a-b, the RFcombination network 50, and the summing circuits 48 a-b. With respect toFIG. 3, like reference numerals are utilized in FIG. 7B whereapplicable. The system 220 of FIG. 7 includes a plurality of masterunits 46 a-b that provide signals to respective remote units 42 a-b witha single Rx antenna each and positioned in respective portions 208 a-bof an indoor environment 224. In particular the indoor environmentexample is illustrated as two rooms 228 a-b separated by a wall 230.

As illustrated in FIG. 7B, the respective portions 228 a-b of the indoorenvironment 224 are somewhat electromagnetically isolated (e.g., lowlevels of a signal sent from a wireless device 232 (device A or B) inone portion 228 detected by the remote unit 42 of another portion 228).In specific embodiments, the respective portions 228 a-b are separatedby the partition or wall 230. This illustration shows the case of goodUplink power isolation between the different remote unites 42 a-b andthe related mobile devices 232 a and 232 b. Therefore the Uplinkmulti-user MIMO feature operates desirably because at the BTS antennaports, the mutual interference of the signals from the two devices 232a-b only depends on the isolation provided by the indoor radio planning.Although isolation will be determined by the deployment of the remoteunits and the location of the users and mobile devices, indoor scenariosoffer good isolation due to the presence of multiple walls and floors.Also, the hybrid coupler of the invention doesn't affect the BTS MIMOdecoder since the signals from the mobile devices 232 a-b areorthogonally cross-coupled to the BTS antenna ports thus avoiding theirmutual interference. Therefore in another aspect of the invention, withtwo fully isolated groups of users served by two remote units connectedto different Master Units, the Uplink Multi-User MIMO feature canachieve a complete reuse of the time/frequency resources of the BTS. Asa consequence the number of users manageable in the Uplink path by theMIMO BTS would be increased and possibly doubled.

It will be appreciated that such an aspect of the invention might be incontrast to the feature discussed herein of maintaining a certain degreeof signal coverage overlapping between Remote Units as requested by theDownlink Single-User MIMO when implemented through DAS. Therefore, forrealizing both such advantages, a tradeoff would have to be consideredand managed to balance the benefits of both the MIMO features. In thiscontext the same 90° 3 dB Hybrid Coupler can be exploited both inDownlink and in Uplink paths of an indoor DAS for MIMO signals.

Thus, each remote unit 42 a-b provides signals to, and receives signalsfrom, respective wireless devices 232 a-b that are present within thoserespective portions 228 a-b. One benefit of this arrangement as noted isthat uplink collaborative MIMO (for WiMAX) and/or uplink multi-user MIMO(for LTE) may be used to increase the total uplink capacity by reusingthe time and/or frequency resources associated with the differentwireless devices 232 a-b.

The invention in its broader aspects is not limited to the specificdetails representative apparatus and method, and illustrative examplesshown and described. Accordingly, departures may be made from suchdetails without departure from the spirit or scope of the applicants'general inventive concept. For example, the system 10 of FIG. 2A, thesystem 40 of FIG. 2B, the system 100 of FIG. 3, and/or the system 220 ofFIG. 7 may be configured with an extension unit (not shown) disposedbetween a master unit 46 and its corresponding remote units 42. Theextension unit may provide additional links for coupling a master unit46 to additional remote units 42 and/or the extension unit may extendthe range of coupling between a master unit 46 and remote units 42.Moreover, the system 10 of FIG. 2A, the system 40 of FIG. 2B, the system100 of FIG. 3, and/or the system 220 of FIG. 7 may be configured withmore or fewer remote units 12 or 42, master units 16 or 46, SISO BTSs 20or 54, MIMO BTSs 30 or 58, system controllers 22 or 76, summing circuits48, RF combination networks 50, and/or hybrid couplers 52, as well assupport more or fewer wireless devices 26, 90, and/or 232 consistentwith embodiments of the invention. Similarly, the system 10 of FIG. 2A,the system 40 of FIG. 2B, the system 100 of FIG. 3, and/or the system220 of FIG. 7 may include a MIMO BTS 30 or 58 with more or fewerantennas 31 and/or 32, a hybrid coupler 52 with more or fewer ports, aswell as a master unit 16 or 46 configured with more or fewer inputs oroutputs consistent with embodiments of the invention.

Additionally, it will be appreciated that the indoor environments 104and 224 of FIGS. 3 and 7B, respectively, are merely included to showoperation of embodiments of the invention therewith, and thatembodiments of the invention may be used with outdoor environmentswithout departing from the scope of the applicants' general inventiveconcept. Moreover, one of ordinary skill in the art will appreciate thatsystem 220 of FIG. 7B may include SISO BTSs 54 a-b as well as the RFcombination network 50 and summing circuits 48 a-b consistent withalternative embodiments of the invention.

Furthermore, in some embodiments, the indoor environment 224 of FIG. 7Bwill be configured in other ways than just including partition 230. Assuch, the respective wireless devices 232 a-b may be isolated in otherways.

FIGS. 10-12 illustrate further alternative systems for incorporatingembodiments of the invention within a distributed antenna system.Specifically, system 250 illustrated in FIG. 10 is an interleaveddistributed antenna system, incorporating MIMO signals, wherein thedeployment of the system is implemented utilizing a combination ofoptical fibers, as well as an RF passive distribution network, for thepurposes of signal distribution. Specifically, optical fiber is used asa link for handling traffic between the master units and remote units,while an RF distribution network is implemented between the remote unitsand one or more passive antennas coupled to the remote units. To thatend, system 250 illustrates a particular distributed antenna system thatincorporates a MIMO BTS 252 for handling signals from one or moreantennas 253, 254. Such a system may take advantage of the presentinvention. Although the embodiments illustrated in FIGS. 10-12incorporate two antennas and two MIMO signals (n=2), as noted herein, aMIMO system of the invention may incorporate additional antenna elementsand signals as well. Therefore, the invention is not limited to theillustrated number of different MIMO antennas or MIMO channels.

The MIMO signals 255, 256, from respective MIMO antennas 253, 254 of theMIMO BTS, are delivered to master unit 260 in a suitable fashion, suchas using an RF network (e.g., Coaxial cables), for distributionthroughout the remote components of the system. Similarly, uplinksignals from wireless devices are delivered to the MIMO BTS through themaster unit 260. Although a single master unit is illustrated in theillustrated figures and discussed herein, the system 250 might utilizeone or more master units.

To that end, the MIMO signals are appropriately processed and deliveredto one or more remote units 262, 264 for further distribution of thosesignals to wireless devices and equipment, such as cellular phones. Asuitable signal link, such as a fiber link 266, 268, is incorporatedbetween the master unit 260 and the various respective remote units. Themaster unit 260 and remote units 262, 264 can handle the multiple MIMOsignals or channels 255, 256 in an appropriate fashion over the fiberlinks 266, 268. For example, frequency conversion might be implementedwherein the multiple MIMO channels (two channels in the illustratedexample) are converted in frequency so that their integrity ismaintained over the fiber links 266, 268. Alternatively, the fiber links266, 268 might incorporate multiple fibers wherein each fiber carriers aseparate MIMO channel signal (Antenna 1/Channel 1 or Antenna 2/Channel2) to maintain the integrity of MIMO signals between the master unit 260and the multiple remote units 262, 264. In still another embodiment,wavelength division multiplexing (WDM) might be implemented on the fiberlinks 266, 268 between the master and remote units to maintain theintegrity of the MIMO channels.

In the system 250 illustrated in FIG. 10, the various signals, includingthe MIMO signals, are then passively distributed from the remote unitsin an RF distribution network out to passive antenna elements 270 a-270d. For example, the remote units 262, 264 might be coupled to theantennas 270 a-270 d through suitable RF links, which might implement RFcables, such as coaxial cables, and suitable power splitters fordelivering the signals to multiple passive antennas 270 a-270 d. In thesystem 250, the MIMO channels are still segregated, and thus, eachparticular antenna unit 270 a-270 d would only handle one of the MIMOchannels.

Referring now to FIG. 11, in accordance with one embodiment of theinvention, a hybrid coupler circuit is implemented in the system 251.Specifically, a hybrid coupler circuit is implemented between each ofthe remote units 262 and 264 and the respective passive antennas 270a-270 d. As illustrated in FIG. 11, the hybrid coupler circuits 274,which may be 90°, 3 dB hybrid RF couplers as discussed herein, are shownsituated between the various remote units and antennas. As illustratedin FIG. 11, each antenna 270 a-270 d then handles a portion of each ofthe MIMO signals in accordance with the invention for communication witha wireless device, such as wireless device 280.

As discussed above, the hybrid coupler circuits are configured toreceive the first MIMO signal (Antenna 1/Channel 1) and the second MIMOsignal (Antenna 2/Channel 2) from the respective remote unit atrespective first and second ports. Output signals are then provided atoutput ports of the couplers, and the output signals include portions ofthe first and second MIMO signals. The output signals, with the combinedMIMO signal portions, are then delivered to the various antenna elements270 a and 270 d and broadcast appropriately to wireless devices in thesignal vicinity. In that way, each antenna in the indoor environmenthandles both or all MIMO channels as illustrated.

FIG. 12 illustrates another alternative embodiment of the invention,wherein a hybrid coupler circuit 274 is implemented within an opticalfiber and RF distribution system 258, similar to that as illustrated inFIG. 10. Rather than incorporating a hybrid coupler circuit with each ofthe remote units, a hybrid coupler circuit 274 is incorporated betweenthe MIMO BTS 252 and the master unit 260. The coupler circuit combinesthe multiple MIMO channels on output ports as discussed and the variousoutput ports are coupled with the master unit 260. The combined MIMOsignals at the master unit are then appropriately directed to the remoteunits 262, 264 utilizing fiber, and then further distributed through anRF link or passive distribution network to the antennas 270 a-270 d, asdiscussed above. Similar to the embodiment in FIG. 11, each antenna inthe indoor environment handles both or all MIMO channels as illustrated.

Other modifications will be apparent to one of ordinary skill in theart. Therefore, the invention lies in the claims hereinafter appended.Furthermore, while embodiments of the invention has been illustrated bya description of the various embodiments and the examples, and whilethese embodiments have been described in considerable detail, it is notthe intention of the applicants to restrict or in any way limit thescope of the appended claims to such detail. Additional advantages andmodifications will readily appear to those skilled in the art. Thus, theinvention in its broader aspects is therefore not limited to thespecific details, representative apparatus and method, and illustrativeexample shown and described. Accordingly, departures may be made fromsuch details without departing from the spirit or scope of applicants'general inventive concept.

What is claimed is:
 1. A system comprising: a multiple-input andmultiple-output (MIMO) signal configured to output a first branch of aMIMO signal and at least a second branch of the MIMO signal, thebranches of the MIMO signal being in the same frequency channel; atleast one master unit and a respective plurality of remote units coupledwith the master unit, the at least one master unit coupled with the MIMOsignal and configured for distributing signals to one or more remoteunits; at least another master unit and another respective plurality ofremote units coupled with the another master unit, the another masterunit coupled with the MIMO signal and configured for distributingsignals to one or more remote units; a hybrid coupler coupled to receivethe first branch of the MIMO signal and the second branch of the MIMOsignal on respective first and second input ports and configured toprovide output signals on first and second output ports, the outputsignal on the first output port including at least a portion of thefirst branch of the MIMO signal and at least a portion of the secondbranch of the MIMO signal that is phase shifted orthogonally in relationto the respective second branch of the MIMO signal and the output signalon the second output port including at least a portion of the secondbranch of the MIMO signal and at least a portion of the first branch ofthe MIMO signal that is phase shifted orthogonally in relation to therespective first branch of the MIMO signal; a plurality of remote unitsconfigured for communicating the output signal of the first output portof the hybrid coupler; another plurality of remote units configured forcommunicating the output signal of the second output port; the systemconfigured for communicating both the output signal of the first outputport and the output signal of the second output port to be used by adevice for providing MIMO communications.
 2. The system of claim 1,wherein the remote units are coupled with respective master units overoptical links.
 3. The system of claim 2, wherein at least one of theoptical links comprises: a first optical fiber for carrying uplinksignals; and a second optical fiber for carrying downlink signals,wherein the first and second optical fibers are configured as halfduplex channels.
 4. The system of claim 2, wherein at least one of theoptical links comprises: an optical fiber for carrying uplink anddownlink signals on different wavelengths; and a wavelength divisionmultiplexer configured to combine or split the uplink and downlinksignals on the optical fiber.
 5. The system of claim 1, wherein at leastone of the remote units communicates with at least one of the masterunits over a high data rate media selected from a group consisting ofcoaxial cable, twisted pair copper wires, a free space radio frequencylink, and a shared network, and combinations thereof.
 6. The system ofclaim 5, wherein the shared network is selected from a group consistingof Ethernet, SONET, SDH, ATM, PDH, and combinations thereof.
 7. Thesystem of claim 2, wherein the optical links carry at least one of amodulated digital signal or a radio-over-fiber (RoF) signal.
 8. Thesystem of claim 1, wherein the hybrid coupler is a 90° 3 dB hybridcoupler.
 9. The system of claim 1 further comprising a SISO basestation, the SISO base station being selectively coupled with at leastone of the master units and providing a SISO output signal.
 10. Thesystem of claim 9, wherein the system is further configured tocommunicate the output signal from the SISO base station simultaneouslywith the output signal of the first output port and the output signal ofthe second output port.
 11. The system of claim 1, wherein at least oneof the plurality of remote units is configured to communicate therespective output signal of an output port to a device with an airinterface that includes at least one of an antenna and a radiatingcable.
 12. A system comprising: a multiple-input and multiple-output(MIMO) base station configured to output at least a first signal and asecond signal; at least one master unit communicating with the MIMO basestation and configured to receive the first signal and second signalfrom the base station; at least one remote unit communicating with themaster unit and configured to receive the first signal and second signalfrom the master unit; at least one antenna coupled with the remote unitfor receiving signals from the remote unit; a hybrid coupler coupledbetween the remote unit and antenna and configured to receive the firstsignal and the second signal from the remote unit on respective firstand second ports and to provide an output signal on at least one outputport, the output signal including at least a portion of the first signaland at least a portion of the second signal; the antenna coupled withthe output port.
 13. The system of claim 12, wherein the remote unitcommunicates with the master unit over an optical link.
 14. The systemof claim 13, wherein the optical link includes at least one of: firstand second optical fibers for carrying uplink and downlink signalsrespectively, or an optical fiber for carrying both uplink and downlinksignals on different wavelengths and a wavelength division multiplexerconfigured to combine or split the uplink and downlink signals on theoptical fiber.
 15. The system of claim 12, wherein the couplercommunicates with the remote unit over an RF link.
 16. The system ofclaim 12, wherein the hybrid coupler is a 90° 3 dB hybrid coupler. 17.The system of claim 12, wherein the coupler is further configured tooutput a plurality of output signals on a plurality of output ports,each of the plurality of output signals including at least a portion ofthe first signal and at least a portion of the second signal.
 18. Thesystem of claim 1, wherein at least one of the portions of the first andsecond signals making up the output signal is phase shifted.
 19. Thesystem of claim 1, further comprising: a plurality of remote unitscommunicating with the master unit.
 20. The system of claim 1, whereinat least one of the portion of the first signal or portion of the secondsignal is phase shifted orthogonally in relation to the respective firstsignal and second signal.